The present invention relates to space heaters and more particularly to a dual-rate fuel control system for space heaters providing improved thermal efficiency and fuel economy.
Warm air, hot water and steam space heaters are now in general use. Such space heaters comprise, according to the type of heat transfer fluid being used, a fan or blower for delivering heated air from a furnace heat exchanger to various rooms or spaces to be heated in a building, or a hot water circulation pump or steam flow valving system for circulating hot water or steam to diverse radiators in the various spaces to be heated. Generally, the heating system includes a control thermostat disposed in one of the spaces to be heated, or a thermostat in each space to be heated co-operating with appropriate individual controls for valving means, blower motors or circulation pump motors. The control thermostat causes the burner of the furnace to start when heat is called for as a result of the temperature dropping below a predetermined "low". As soon as the heat transfer fluid in the heat exchanger chamber reaches a predetermined temperature, a fan or blower in warm air systems, a circulation pump in hot water systems or a valve in steam systems, is automatically activated for circulating heated air through the rooms or spaces to be heated, or for circulating hot water or steam through the appropriate radiators. When the temperature in the room or spaces to be heated reaches a predetermined "high" temperature, the thermostat automatically shuts off the supply of fuel to the furnace burner, but the heat transfer fluid continues to circulate until the temperature of the fluid in the heating chamber, or in the boiler, has dropped below a predetermined temperature.
Dual-range heating systems have been proposed in the past. For example, U.S. Pat. No. 2,693,914 discloses a warm air furnace system in which the burner has a high setting and a low setting, and in which part of the air heated during operation of the burner at low setting is by-passed through the heating chamber. U.S. Pat. No. 2,800,282 discloses a dual burner and a control system for operating only one burner when the outside temperature is above a predetermined temperature, and for operating both burners when the outside temperature is below the pre-determined temperature. U.S. Pat. No. 2,266,563 teaches an arrangement for monitoring the temperature in a hot air duct such as to reduce the heat input from the burner when the temperature in the duct becomes excessive, and such as to return the burner to full capacity when the temperature in the duct drops. U.S Pat. No. 3,486,693 discloses a modulating fuel flow control providing a variation or modulation of the flow of fuel to a burner as a function of the variation of temperature in the space to be heated, while U.S. Pat. No. 3,999,934 discloses a thermostat control which provides full fuel flow to a burner during start-up and which reduces the flow of fuel to the burner after start-up.
Although attempts have thus been made in the past to effectuate fuel economy and energy saving in space heating systems, such attempts have apparently not met with great commercial success in view of their relative complication, sometimes coupled with lack of reliability and high cost of installation. The present invention, by contrast, derives from the observation that it is more efficient to transfer small amounts of heat through a heat exchanger than to transfer large amounts of heat through the same heat exchanger and that, after start-up of the heating system, a smaller amount of heat being supplied to the heat exchanger is only required for maintaining the system in an efficient mode of operation.